1. Field of the Invention
Apparatuses and methods consistent with the present invention relate to correcting frequency characteristics of a carrier chrominance signal, which, by use of the multi burst signals involved in a vertical blanking interval, determine whether the frequency characteristics of the carrier chrominance signal are distorted and correct the frequency characteristics.
2. Description of the Related Art
A broadcasting receiver is required for reception of analog broadcasting including a National Television System Committee (NTSC) system, a Phase Alternation Line (PAL) system, and a Sequential Couleur Avec Memoire (SECAM) system. Since the recent launch of the digital broadcasting, a simulcast that broadcasts the same contents as those of a digital broadcast has continued in an analog manner in order to provide service for users who have an analog broadcasting receiver. Thus, it is true that a digital broadcasting receiver also has a receiving end for receiving analog broadcasts.
When an analog broadcasting channel is selected, a tuner receives the corresponding radio frequency and transforms it to a 45.75 MHz intermediate frequency video signal. The intermediate frequency video signal is demodulated at a video detector circuit into a composite video signal, called CVBS (color, video, burst, sync) signal, that the intermediate frequency video signal is separated therefrom. The composite video signal is input into a video decoder and thus decomposed into three primary color signals for practical color reproduction. Then, the decomposed video signal is displayed through a display unit such as CRT and so on.
The composite video signal is shaped in such a manner that a 3.58 MHz (accurately, 3.579545 MHz) color subcarrier is balance-modulated and sidebands thereof are combined to form a carrier chrominance signal, and the luminance signal is then combined thereto. However, the overlapping of the carrier chrominance signal to the luminance signal renders an amplitude of the carrier chrominance signal excessively enlarged, providing over modulation, so that it requires a compression of the amplitude. Thus, the carrier chrominance signal is compressed in its amplitude and is combined with the luminance signal.
A video decoder of the broadcasting receiver includes therein an analog-digital converter and a comb filter so as to receive an analog composite video signal that the luminance signal and the color signal are multiplexed and input thereto. An analog composite video signal is converted into a digital signal by the analog-digital converter, which is in turn decomposed into the luminance signal and the color signal by the comb filter. If frequency characteristics of the decomposed color signal are distorted, a certain correction is necessary.
FIGS. 1A to 1C are graphs illustrating the frequency characteristics of the composite video signal in order to explain a problem of the prior art.
A graph of FIG. 1A shows the frequency characteristics of the normally received composite video signal. The frequency component 101 of the luminance signal reaches up to 4.2 MHz, and the frequency component 103 of the color signal is interleaved into the luminance signal while being carried in the 3.58 MHz color subcarrier 105.
Such frequency characteristics of a composite video signal are distorted in accordance with broadcast channels or the type of the broadcasting receiver.
FIG. 1B shows the frequency characteristics of a composite video signal of FIG. 1A being distorted. The distortions may be caused by the frequency characteristics of the broadcast channels or the broadcasting receiver. Referring to a graph of FIG. 1B, one can understand that signals of high frequency band are mainly distorted. Practically, the signal distortion in a 4.2 MHz high frequency band indicates that the color signal is more distorted than the luminance signal.
Upon decomposing the input video signal with such frequency characteristics as shown in FIG. 1B into the color signal by use of the comb filter, a graph as shown in FIG. 1C appears.
FIG. 1C shows the distorted frequency characteristics of the color signal carried in the color subcarrier.
In order to correct the distortion of such frequency characteristics, the color signal from the comb filter is corrected with a constant fixed value using an active band pass filter. However, such correction irrespective of a degree of the distortion cannot be an accurate correction.